Add.
Answer: There are many ways to solve this problem. Let's see two ways we can solve it. Place value table Let's line up the numbers by their decimal places. Tens Ones $.$ Tenths Hundredths $2$ ${1}$ $.$ ${9}$ $0$ $5$ $6$ $.$ ${5}$ $1$ Now, let's add each place value column. Tens Ones $.$ Tenths Hundredths $2$ $\overset{1}{{1}}$ $.$ ${9}$ $0$ $+$ $5$ $6$ $.$ ${5}$ $1$ $\underline{~~~~}$ $\underline{~~~~}$ $\underline{~~~~}$ $\underline{~~~~}$ $7$ $8$ $.$ $4$ $1$ Decomposing the numbers We can group the whole numbers together and the decimals together. $\begin{aligned} &({21}+ {56}) + ({0.90} + {0.51})\\\\ &=77 + {1.41}\\\\ &=78.41 \end{aligned}$ $78.41 = 21.9 + 56.51$